Grain oriented electrical steel sheets, which are mainly used as iron cores of transformers, are required to have excellent magnetic properties, in particular, less iron loss. To meet this requirement, it is important that secondary recrystallized grains are highly aligned in the steel sheet in the (110)[001] orientation (or so-called “Goss orientation”) and impurities in the product steel sheet are reduced. However, there are limitations to control crystal orientation and reduce impurities in terms of balancing with manufacturing cost, and so on. Accordingly, there have been developed techniques for iron loss reduction, which is to apply non-uniform strain to a surface of a steel sheet physically to subdivide magnetic domain width, i.e., magnetic domain refining techniques.
For example, JP 57-002252 B proposes a technique for reducing iron loss of a steel sheet by irradiating a final product steel sheet with a laser, introducing a high dislocation density region to the surface layer of the steel sheet and reducing the magnetic domain width. In addition, JP 62-053579 B proposes a technique of refining magnetic domains by forming linear grooves having a depth of more than 5 μm on the steel substrate portion of a steel sheet after being subjected to final annealing at a load of 882 MPa to 2156 MPa (90 kgf/mm2 to 220 kgf/mm2), and then subjecting the steel sheet to heat treatment at a temperature of 750° C. or higher. Moreover, JP 3-069968 B proposes a technique of introducing linear notches (grooves) of 30 μm to 300 μm wide and 10 μm to 70 μm deep, in a direction substantially perpendicular to the rolling direction of a steel sheet, at intervals of 1 mm or more in the rolling direction.
With the development of the magnetic domain refining techniques as above, it is now becoming possible to obtain grain oriented electrical steel sheets having good iron loss properties.
Usually, however, in the case of using a technique of forming grooves on a surface of a steel sheet, there is a tendency that the coating is applied more heavily to the floors of grooves due to the liquid flowing into the grooves from their circumference while the coating is being applied. This results in larger differences in coating film thickness between the grooves and portions other than the grooves. Consequently, there is a problem of a non-uniform distribution of the tension applied by the coating, causing strong local stress to be exerted on the grooves. Further, any external stress applied due to sheet passage through a manufacturing line or the like would be unsustainable for those portions to which local stress has already been applied as described above, thereby causing partial exfoliation and defects of the film. Such defects pose problems associated with deterioration in corrosion resistance as well as loss of insulation resistance.
It could therefore be helpful to provide such a grain oriented electrical steel sheet that may reduce local exfoliation of insulation coating films and has excellent corrosion resistance and insulation properties.